Evangelos and Evaggelos are not two different transliterations (the exact transposition of letters from an alphabet to another one: the transliteration is exact and identic for all the languages and only varies in the transliterative system, if there are several systems for one alphabet, and it is mainly used for scholar works), but one Latinized form (Evangelos) from a Greek one (Evaggelos, in Greek Ευάγγελος), probably attracted by the Latin word evangelium or by some names as Evangeline, in English, or Evangelista, in Spanish (both from the Latin form).

In fact, in Greek, the search with Google for Ευάνγελος, which would be the Greek spelling of Evangelos, offer only 6 results (2 Spanish, 1 Italian, 2 Greek and 1 English about Vangelis) versus 607,000 of Ευάγγελος. And, for example, the one about Vangelis is not correct checking with the informations about him in Wikipedia.

http://www.search.com/reference/Vangelis"Evangelos Odysseas Papathanassiou (Greek: Ευάνγελος Οδυσσέας Παπαθανασίου IPA: [evˈaɲɟelos oðiˈseas papaθanaˈsiu]) is a Greek composer of electronic, new age and classical music and musical performer, under the artist name Vangelis Papathanassiou (Βανγέλης Παπαθανασίου) or just Vangelis (a diminutive of Evangelos) [IPA: /væŋɛlɪs/ or /vægɛlɪs/]."

But in English Wikipedia:"Evangelos Odysseas Papathanassiou (born March 29, 1943) (Greek: Ευάγγελος Οδυσσέας Παπαθανασίου IPA: [evˈaɲɟelos oðiˈseas papaθanaˈsiu]), is a Greek composer of electronic, progressive, ambient and neo classical music, under the artist name Vangelis (a diminutive of Evangelos; pronounced /vænˈgɛlɨs/ in English)."

And, more important, in Greek Wikipedia:"Βαγγέλης Παπαθανασίου" and "Ευάγγελος Οδυσσέας".

So, it seems that the form Ευάνγελος in reference to Vangelis is, simply, a mistake and that even if the form is not completely unknown in Greek, its presence is punctual.

They could be two transcriptions from the same word, because that is the change of a word spelled in one alphabet to another alphabet, matching with the specific conventions for a language and trying to reflect the sounds, so the same word is transcript differently in every language of reception. I don't know if the Latin evangelium is simply an adaptation from the Greek εὐαγγέλος or there was a variant εὐανγέλος in this language.